While potentially useful in a wide variety of applications, the present invention evolved in the field of optical libor fabrication. Optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. Optical fibers are typically manufactured by constructing an optical fiber preform of appropriate composition and drawing fiber from the preform.
A typical preform is in the form of a concentric glass rod having a length of about one meter and an outer diameter of 20-200 mm. The inner core of the rod is a high purity, low loss glass such as germanium silicate glass having a diameter of about 1-5 mm. The concentric outer cylinder, referred to as cladding, is a layer of glass with a lower index of refraction than the inner core.
In conventional manufacture of an optical fiber preform, the core is manufactured as a solid doped silica glass rod within the cladding. An outer jacket of silicate glass, referred to as the overcladding, is then added around the rod in order to provide the desired geometry for fiber draw. Specifically, the rod is placed within the overcladding tube, and the rod and overcladding are laminated to form a fiber optic preform by heating the assembly with an oxyhydrogen torch.
A difficulty with this process is that the lamination introduces in the outer periphery of the overcladding OH impurities which can be deleterious to a drawn fiber. As a consequence it is typically necessary to etch away the outer 0.5 mm of the preform with a plasma torch to eliminate the surface contaminated by the oxyhydrogen torch. Accordingly, there is a need for an improved method and apparatus for laminating overcladding and core rod in fiber optic preforms.